The Joker's on again off again girlfriend/side kick. When she's not with The Joker she can probably be found with Poison Ivy or Red as refered to by Harley. She used to be a clinical phychiatrist at Arkham Asylum until she fell in love with her patient The Joker. Ever since then she's led a life of crime, and has been in and out of Arkham herself. She refers to The Joker as Puddin or Mista J.

Joker: Harley where are my grenade marbles!
Harley Quinn: You left em in the car Puddin!

Another term for a "Mary Sue", stemming from Batman the Animated Series, in which Harley Quinn was an obvious Mary Sue created by Paul Dini to sublimate his sexual desire for the Joker and to make "the Joker appear less gay".

Any character created or written into fan fiction, that clearly does not belong, but is there to sublimate the authors sexual desire for an actual well established character.